1Technical Field
This invention relates generally to liquid toners of the type used in electrophotography. More specifically, the invention relates to polymeric toner particles containing dye monomers, linking/spacing components and dye/charge director components.
2. Background Art
In electrophotography, a latent image is created on the surface of a photoconducting material by selectively exposing areas of the charged surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity, but different levels of charge.
In prior art electrophotographic processes the toner may be in the form of a dust, or powder, having pigment particles imbedded in a particulate resinous carrier, as described, for example in Giaimo, U.S. Pat. No. 2,786,440, issued Mar. 26, 1957. These toner particles may be fused or fixed to the surface by known means such as heat or solvent vapor, or they may be transferred to another surface to which they may similarly be fixed, to produce a permanent reproduction of the original pattern.
Generally, toners for electrophotography (EP) must perform several functions. They must provide color, including black, they must possess a fixed or at least locally stabilized charge in order to respond to the driving force of the electrical field in the development gap, and for liquid systems, toners must form a dispersion in the suspending liquid medium at least during image development. In addition, it is usually desirable for the toner be fusible to the paper (or other receiving medium) in order to obtain permanence of the transferred image.
The related art shows these functions being served by three different chemical species in EP toner systems:
1. a pigment for coloration; PA1 2. a charge direction agent, either loosely associated via Van der Waals attractions or complexed to functional groups on the polymeric resin, (item 3, next); and PA1 3. a polymeric resin, which in liquid toner systems provides steric stabilization of the pigment in the dispersing liquid medium, and for both dry powder and liquid toners is usually fusible to paper or other receiving hard copy.
Traditional EP toners, as described for example, in ELMASRY et al U.S. Pat. No. 4,925,766, ELMASRY, U.S. Pat. No. 4,946,753, and JONGEWAARD, U.S. Pat. No. 4,988,602, have several undesirable characteristics which the present invention resolves. Traditional toners are pigment based. This means that the colorant is present in relatively large, solid particles, on the order of approximately 0.1 to 50 .mu.m. For dry powder systems, the particle size is toward the high end of this range, and for liquid toner systems, smaller particle sizes are achievable. The pigment particle size establishes fundamental physical limits on color quality due to light scatter, and in the case of liquid toner systems, fundamental limitations on mobility of the particle within the fluid medium, which then limits rate of deposition of the toner and EP process and printing speeds.
In Ong et al., U.S. Pat. No. 4,778,742 is described a group of polymeric anthraquinone dyes. However, these dyes are incorporated in resin particles by the prior art techniques.
Light is scattered from particles when the particle dimensions approach, on order of magnitude, the wavelength of the light. For light in the visible range, particles from about 0.3 .mu.m and larger will scatter the visible light. This scatter impacts directly on three areas: the ability to layer color planes over one another on the photoconductor surface, the color quality of projected images such as overhead transparencies, and the reflectance color quality of secondary colors where one color is printed over the top of another color, as observed in both reflectance and transmission modes.
If the pigment particles are of sufficient size, they may interfere with laser discharge of the photoconductor. This makes it increasingly difficult to print a second color over the first, from a process point of view. Secondly, once the color is printed, the quality of that color is impacted if the top layer is scattering the incident light. Therefore, far more of the bottom color, or conversely far less of the top color will have to be printed to compensate for the scattered light. This requires complex printing algorithms to correct for the amount of each colorant deposited, and may require this amount to vary depending on whether a primary or a secondary color is to be printed. This scatter is hue dependent, and that means it is not quantitatively the same for the three primaries. Thus, algorithms to correct for it must be individualized for the three primaries. This adds a level of complexity to the firmware required to operate the printer. Furthermore, the quality of projected transparencies can never be as good as if the colorant particle dimensions were well below the wavelength of visible light.
There is a semantic problem with the definition of dyes and pigments, as they are defined by their solubility in the liquid medium. The terminology derives from antiquity when most dyeing was done in aqueous media. A "dye" is soluble (generally speaking of aqueous solutions) and a "pigment" is not soluble. What is a "dye" in one solvent system may become a "pigment" when the solvent is changed. Many toner fabricators have simply overlooked dyes in their search for new colorants, without recognizing that many "dyes" would behave as "pigments" in the nonaqueous environment of the EP toner.
One objection in the past to using dyes in toners is the erroneous perception that dyes lack sufficient lightfastness or tinctorial strength to be used in printer applications. This objection is easily addressed by a survey of lightfast dyes. Many dyes exist which have excellent lightfastness, and some are even used in the standard comparative lightfastness tests such as DIN 54 003 and DIN 54 004.
The prior art teaches two methods of preparing liquid EP toners, both of which are based on relatively large pigment particles. One traditional method is to melt the polymeric resin and blend finely ground pigment into the melt. The blend is cooled to allow it to solidify, and then pulverized to a fine powder. The powder is subsequently dispersed in a non-conducting non-polar liquid such as a petroleum distillate. The second method is to prepare a hydrocarbon (or other solvent) dispersion of the polymeric resin and blend the finely ground pigment into the dispersion. During the blending process, the polymeric resin comes into contact with the pigment particle and forms an attachment to the pigment via Van der Waals forces, dipole-dipole interactions or even covalent bonding. These fabrication methods involve many steps, and do not lend themselves well to fully automated assembly line processes. Furthermore, both methods begin with a pigment particle in the size range of 0.05 .mu.m to 2/.mu.m or even greater, and build up a resin coating on that. The final particle size can be 0.2 to 70 .mu.m or even greater, so these EP toners tend to flocculate and eventually settle. Also, particles in this size range possess the above-described, disadvantageous property of light scattering.
Most attempts to eliminate the scatter due to relatively large particle size have involved one or both of two possible approaches. The resin-coated pigment particle can be reduced in size, or the pigment itself can be ground very fine prior to binding the resin to it, and then the thermoplastic resin coating can be fused at any of several points in the EP process to produce an amorphous phase in which the pigment is dispersed. With either approach, simply milling or grinding the pigment or dye chosen to a finer particle size will help. There are disadvantages to simply milling or grinding to a finer particle size in both dry powder and liquid systems, including increased cost of the raw materials, as finely ground pigments and soluble dyes are invariably more expensive than the coarser size particle of the same substance.
In dry powder electrophotographic systems, the toner is triboelectrically charged, and attracted through air via electrostatics to the photoconductor surface where the image is formed. There are fundamental limitations on how small these particles can be and still maintain control of their location in space. If the particles are below about 5 .mu.m, they become difficult to control via electrostatics alone. The result may be increased background scatter and decreased homogeneity of solid fill areas in the printed image. An additional disadvantage of these very small-sized particles is the associated health and environmental concerns. Any dust, however chemically inert, that falls in this size range, is small enough to pass by the nasal hairs and mucosa and enter the lungs, yet not small enough to be simply exhaled again. That means particles of this size must absolutely be contained within the printer, for regulatory and ethical reasons, and this again complicates the printer design, and increases its cost.
One solution to this difficulty is to contain the preferred smaller particles in a liquid dispersant, and a number of liquid toner systems have been reported. All of them employ steric and/or electrostatic means to stabilize the relatively large dispersed particle in a non-polar, non-conducting medium such as petroleum distillate. The liquid toners reported to date have been dispersions of particles on the order of 0. 1 .mu.m to 50 .mu.m in diameter. The disadvantage of these dispersions is that they are often difficult to stabilize, particularly at the high end of this diameter range where gravity exerts enough force to overcome the tendency of the particles to repel each other, and thus promotes settling. Some toners are composed of finely ground pigment particles with very little resin binder attached. In these toners, the particle size may be very small, but they lack the steric stabilization afforded by adding a polymeric resin. The resin provides the steric stabilization but necessarily increases particle size.
In all these approaches, the fundamental size of the ground pigment itself causes light scatter, and can disrupt the laser beam making overprinting of one color on another difficult or impossible. To solve the problem of nontransparent color planes, various print methods are used. One way is to lay the primary color planes down one at a time on the photoconductor, and to remove them to an intermediate holding material until they can all be transferred to paper, which is indirect transfer. Indirect transfer requires additional mechanical and electronic systems, increasing cost and complexity of the printer. Each transfer step increases the probability that errors in alignment or registration may occur. Direct transfer of the color planes one at a time to the paper, or other final receiving medium, is conceptually simpler, but requires the paper or other medium to be manipulated back and forth through the developer several times, and is prone to errors and difficulties, including saturation of the paper with the dispersing medium of the toner.
Print algorithms have been employed to accomplish more simply with software what is cumbersome and difficult using hardware. One way to get around the problem of non-transparent toners while developing all four color planes on the photoconductor at the same time is to lay primary colors side by side. This eliminates the need for multiple passes of the paper or intermediate transfer material, improves color-to-color registration, and works by allowing the human eye to integrate the primaries in order to perceive the desired secondary and tertiary colors. However, there is a finite number of colors which can be produced in this way, and if there is imperfect color-to-color registration, the number of colors is reduced even further. An additional and significant disadvantage is that the effective resolution of the printer is reduced. With only 2.times.2 pixelling arrangements, which is the minimum size, a 600.times.600 dpi printer is reduced to a resolution of 300.times.300, and without gray scaling capability only about 300 colors can be made. Regardless of what is done to improve color on paper, there remains in any case the problem of overhead transparencies that are printed with pigments that are large enough to scatter visible light and therefore are not quite transparent. Colors projected look dark and muddy, not bright and clear. The effect may be especially objectionable in the case of yellow, which appears brown or blotchy when the particle size is large enough to scatter.
A further problem with the relatively large particle size is that the print quality is degraded, in terms of resolution, edge roughness, edge sharpness, and background scatter. Scatter, when it occurs, is far more noticeable and objectionable on the printed page because individual particles are often large enough to be visible with the unaided human eye. Smaller particles, unresolved individually, contribute to a grey or pastel background.
Larger particles also have lower mobility in the dispersing medium. For the same density of charge bearing locations within a particle, surface charge-to-mass decreases as the particle diameter increases. At the same time mobility is decreasing, due to drag forces. The net effect is a much slower migration of the charged particles toward the photoconductor. In practical terms this means either lower print density on the paper or longer developer residence times, slower print times and fewer pages per minute.
Fusing or melting the polymeric resin in which the pigment is embedded, after developing the image, is almost universally employed in order to convert the discrete toner particles into an amorphous film. This is not enough to overcome the effects of scatter when the size of the fundamental pigment particles, embedded in the amorphous film, is on the order of magnitude of the visible light. Smaller particles are needed, but are not easily obtained or stabilized in dispersion using the current technology.
It is therefore an object of the present invention to provide a single macromolecule that consolidates all of the functions served by the toner particle.
A second object of this invention is to provide a smaller sized toner particle that allows faster print speeds due to reduced drag, and a possibility of greater charge to mass ratio (q/m) and greater electrophoretic mobility.
It is an additional objective of this invention to provide a smaller sized toner particle to reduce or overcome the effects of scatter of light in the visible range.
A further objective of this invention is to provide a smaller sized toner particle to allow superior print quality, not limited by the toner particle dimension.
It is a further objective of this invention to provide a liquid toner that forms more stable dispersions, and possesses a longer shelf and storage life.